1. Field of the Invention
The present invention relates to an exposure apparatus, and more particularly, to a projection exposure apparatus, which projects and exposes a semiconductor circuit pattern or a liquid crystal display element pattern on the reticle onto a photosensitive substrate.
2. Discussion of the Related Art
Semiconductor integrated circuits or the liquid crystal displays are commonly manufactured by repeating several times or more the processes of film formation, circuit pattern exposure, etching, and the like. For this reason, a projection exposure apparatus needs to align an exposing pattern on a reticle with the existing pattern on a photosensitive substrate (such as a semiconductor wafer or a glass plate coated with a photoresist layer) with high accuracy upon exposure. To achieve this, the projection exposure apparatus detects the position (and rotation) of the photosensitive substrate (wafer) and the position of the circuit pattern on the reticle using various alignment sensors at the time of the pattern exposure.
The optical systems of these alignment sensors are set at a predetermined position with respect to a reference point in the exposure area of the projection lens PL (for example, the center of the exposure field). The predetermined position may, however, deviates from the design value due to errors in manufacture, temperature variation, and changes in dimensions that may occur over time. When the distance between this reference point (center of exposure) and a reference point in the detection area of the alignment sensor (this distance or positional relationship is referred to as baseline amount or baseline length) is not known exactly, alignment accuracy suffers. Thus, the projection exposure apparatus is equipped with a fiducial mark FM installed at the same height as the wafer to measure the baseline amounts for the various alignment optical systems under the condition that the surface of the fiducial mark FM is matched with the best imaging plane (conjugate plane to the reticle) of the projection lens PL. The baseline amount is measured every time the reticle is replaced by a new one and before the first pattern is exposed using the new reticle.
Additionally, due to the several times or more of the repeated processes, such as film formation, circuit pattern exposure, etching, and the like, the surface of wafer typically has an area where several layers of films are laminated and an area where no films are laminated in the course of such processes. The thickness of one layer is about 0.1.about.1 .mu.m, and thus the level difference in the wafer surface (within one shot area) may become as much as a few .mu.m.
On the other hand, the focal depth .DELTA.Z of the projection lens PL is in general expressed by .+-..lambda./(2.multidot.NA.multidot.Na), where .lambda. is the wavelength of the exposure illumination light, and NA is the numerical aperture of the projection lens on the image plane side. The projection lens PL having NA about 0.5 has been manufactured for use of a KrF excimer laser (.lambda.=0.248 .mu.m), and the focal depth .DELTA.Z for this lens is approximately .+-.0.50 .mu.m (width 1.0 .mu.m). When exposure is performed on the wafer that is positioned to the best imaging plane of the projection lens and the level difference on the wafer is a few .mu.m, the surface level difference of the wafer exceeds the focal depth of .+-.0.50 .mu.m of the best imaging plane (best focal plane). In this case, image formation becomes impossible. This is a problem especially when exposing contact holes.
To solve this, Japanese Laid-Open Publication No. 63-42122 discloses an apparatus which increases an effective focal depth by performing multiple exposure of the same reticle pattern while elevating the wafer to two to three positions in the direction parallel to the optical axis of the projection lens PL (multiple level exposure). In addition, Japanese Laid-Open Publication No. 5-13305 discloses an apparatus which magnifies the effective focal depth 1.5 to 2 times by exposing the same reticle while moving the wafer W by an amount of .+-..DELTA.Z (focal depth) at a variable speed in the Z direction of the projection lens.
There are, however, cases where the optical axis of the projection lens PL is not precisely telecentric in the image space between the projection lens PL and the wafer W. For example, FIG. 5 shows the case where the optical axis OA of the projection lens PL is bent (or curved) in the shape of a left bracket "&lt;."
Suppose that the projection lens PL has the depth of focus (DOF) .DELTA.Z (e.g., about 1.0 .mu.m) and contact holes are exposed by moving the wafer in the Z direction (optical axis direction) relative to the best-focus position Zc. Then, as shown in FIG. 6, which is a magnified view of a portion of FIG. 5 adjacent the wafer, the optical axis OA deviates from the ideal telecentric optical axis IOA by an amount .delta.1 at the best-focus position Zc. On the other hand, when moving the wafer W in the Z direction from the position that is the focal depth amount lower than the best focus position (Zc-.DELTA.Z) to the position that is the focal depth amount higher than the best focus position (Zc+.DELTA.Z), the average displacement amount becomes .delta.2. As shown in the figure, in the case of the bent (or curved) optical axis OA, .delta.1 and .delta.2 do not coincide with each other.
In addition, there are also cases where an optical axis forming the reference point in the detection area of an alignment sensor also bends into the shape of a left bracket "&lt;" between the position Zc-.DELTA.Z (which is a focal depth amount lower than the best focus position) and the position Zc+.DELTA.Z (which is a focal depth amount higher than the best focus position).
Accordingly, if the baseline amount is measured only at the best focus position of the projection lens, alignment errors occur when exposing contact holes by moving the wafer in the Z direction (optical axis direction). In addition, not only in measurement of the baseline amount, but also in an enhanced global alignment (EGA), which statistically processes the positions of alignment marks in a plurality of predetermined shot areas in the wafer to determine the position of the entire wafer, these alignment errors are being included in the result.